Key Points We identify gain-of-function mutations involving IL2RG, JAK1/3, and STAT5B as well as deleterious mutations affecting EZH2, FBXW10, and CHEK2 in T-PLL. Pharmacologic targeting of primary T-PLL cells with the STAT5 inhibitor pimozide leads to apoptosis.

Sézary syndrome (SS) is an aggressive leukaemia of mature T cells with poor prognosis and limited options for targeted therapies. The comprehensive genetic alterations underlying the pathogenesis of SS are unknown. Here we integrate whole-genome sequencing (n=6), whole-exome sequencing (n=66) and array comparative genomic hybridization-based copy-number analysis (n=80) of primary SS samples. We identify previously unknown recurrent loss-of-function aberrations targeting members of the chromatin remodelling/histone modification and trithorax families, including ARID1A in which functional loss from nonsense and frameshift mutations and/or targeted deletions is observed in 40.3% of SS genomes. We also identify recurrent gain-of-function mutations targeting PLCG1 (9%) and JAK1, JAK3, STAT3 and STAT5B (JAK/STAT total ∼11%). Functional studies reveal sensitivity of JAK1-mutated primary SS cells to JAK inhibitor treatment. These results highlight the complex genomic landscape of SS and a role for inhibition of JAK/STAT pathways for the treatment of SS.

The polycomb group (PcG) protein, enhancer of zeste homologue 2 (EZH2), is overexpressed in several human malignancies including breast cancer. Aberrant expression of EZH2 has been associated with metastasis and poor prognosis in cancer patients. Despite the clear role of EZH2 in oncogenesis and therapy failure, not much is known about chemotherapeutics and chemopreventive agents that can suppress its expression and activity. Here, we show that dietary omega-3 (omega-3) polyunsaturated fatty acids (PUFAs) can regulate the expression of EZH2 in breast cancer cells. The treatment of breast cancer cells with omega-3 PUFAs, but not omega-6 PUFAs, led to downregulation of EZH2. Studies using proteosome inhibitor MG132 suggested that omega-3 PUFAs induce degradation of the PcG protein EZH2 through posttranslational mechanisms. Furthermore, downregulation of EZH2 by omega-3 PUFAs was accompanied by a decrease in histone 3 lysine 27 trimethylation (H3K27me3) activity of EZH2 and upregulation of E-cadherin and insulin-like growth factor binding protein 3, which are known targets of EZH2. Treatment with omega-3 PUFAs also led to decrease in invasion of breast cancer cells, an oncogenic phenotype that is known to be associated with EZH2. Thus, our studies suggest that the PcG protein EZH2 is an important target of omega-3 PUFAs and that downregulation of EZH2 may be involved in the mediation of anti-oncogenic and chemopreventive effects of omega-3 PUFAs.

EZH2 (enhancer of zeste homolog 2) is a critical enzymatic subunit of the polycomb repressive complex 2 (PRC2), which trimethylates histone H3 (H3K27) to mediate gene repression. Somatic mutations, overexpression and hyperactivation of EZH2 have been implicated in the pathogenesis of several forms of cancer. In particular, recurrent gain-of-function mutations targeting EZH2 Y641 occur most frequently in follicular lymphoma and aggressive diffuse large B-cell lymphoma and are associated with H3K27me3 hyperactivation, which contributes to lymphoma pathogenesis. However, the post-translational mechanisms of EZH2 regulation are not completely understood. Here we show that EZH2 is a novel interactor and substrate of the SCF E3 ubiquitin ligase β-TrCP (FBXW1). β-TrCP ubiquitinates EZH2 and Jak2-mediated phosphorylation on Y641 directs β-TrCP-mediated EZH2 degradation. RNA interference-mediated silencing of β-TrCP or inhibition of Jak2 results in EZH2 stabilization with attendant increase in H3K27 trimethylation activity. Importantly, the EZH2(Y641) mutants recurrently implicated in lymphoma pathogenesis are unable to bind β-TrCP. Further, endogenous EZH2(Y641) mutants in lymphoma cells exhibit increased EZH2 stability and H3K27me3 hyperactivity. Our studies demonstrate that β-TrCP has an important role in controlling H3K27 trimethylation activity and lymphoma pathogenesis by targeting EZH2 for degradation.

Polycomb group PcG) proteins are overexpressed in several human malignancies including breast cancer. In particular, (aberrant expression of BMI1 and EZH2 has been linked to metastasis and poor prognosis in cancer patients. At present, very little is known about the pharmacological inhibitors of PcG proteins. Here we show that histone deacetylase inhibitors (HDACi) downregulate expression of BMI1. Treatment of MCF10A cells, which are immortal non-transformed breast epithelial cells, and breast cancer cells with HDACi led to decreased expression of BMI1. We further show that downregulation of BMI1 by HDACi results due to the transcriptional downregulation of BMI1 gene. Specifically, we show that primary transcription and promoter activity of BMI1 is suppressed upon treatment with HDACi. Furthermore, downregulation of BMI1 was accompanied by a decrease in histone 2A lysine 119 ubiquitination (H2AK119Ub), which is catalyzed by BMI1 containing polycomb repressive complex 1. HDACi treatment also led to derepression of growth inhibitory genes and putative tumor suppressors, which are known to be silenced by PcG proteins and polycomb repressive complexes (PRCs). In summary, our findings suggest that BMI1 is an important therapy target of HDACi, and that HDACi can be used alone or in combination with other therapies to inhibit growth of tumors that overexpress PcG proteins such as BMI1.